An organic diode has been fabricated which makes use of the electrical conductivity of a low-density polyethylene (LDPE) [L. Boundou, J. Guastavino, N. Zouzou, and J. Martinez Vega, Influence of Annealing Treatment on the Electrical Conductivity of Low Density Polyethylene, Polymer International, 50(9), 1046–1049 (2001), incorporated herein by reference]. This paper reports on the conductivity of semicrystalline, additive free (undoped) low-density polyethylene (LDPE) samples in the form of 100 μm thick discs prepared from hot pressed LDPE pellets. Both faces of each sample were coated with gold by vacuum deposition. The samples were studied by measuring the current through the structure during a stepwise increase in the applied voltage (i.e., in the electric field) at various temperatures. The results of experiments showed evidence of annealing-induced structural changes in the samples and related changes in the distribution of trapping centers. The structure of the samples was modified by temperature variations within the range used to study the conduction mechanism.
Another diode structure was fabricated [L. S. Roman and O. Inganas, Charge Carrier Mobility in Substituted Polythiophene-Based Diodes, Synthetic Metals, 125, 419–422 (2002), incorporated herein by reference] employing the semiconducting polymer poly[3-(2′-methoxy-5′-octylphenyl)-thiophene] (PMOPT) as the active layer material. The layer of the oxidized conducting polymer poly(3,4-ethylenedioxythiophene) doped by poly(4-styrenesulfonate) (PEDOT-PSS) is used as the anode. The diode was designed in the sandwich geometry with 6–10 mm active area. A copper film was deposited by evaporation onto half of a substrate. The PEDOT-PSS layer was deposited by spin coating from a 30% aqueous isopropanol solution filtered via 1-μm pore glass filter and patterned on copper. Then, the deposit was annealed during 5 min at 120° C. in order to remove residual water from the film. The semiconducting polymer was deposited by spin coating from a chloroform solution with a concentration of 5 mg/ml, to a layer thickness of 100 and 200 nm. The second electrode (Al) was vacuum deposited through a shadow mask defining the active area.
The current-voltage characteristic of the diode with a Cu/PEDOT-PSS anode was asymmetric with respect to reverse and forward bias, with a rectification range over five orders of magnitude. The capacitance versus voltage (C-V) characteristics were measured in the dark in the range of applied voltages from −3 to 3 V at a frequency of 50 kHz. The C-V characteristics of these devices agreed with the behavior of the current density and did not show evidence of a depletion layer (Schottky-type characteristics). For the reverse-biased diode with the Cu/PEDOT-PSS anode, the capacitance was constant at a forward voltage of up to 1.5 V and then continuously increased up to 3 V. It was suggested that the dependence of the carrier mobility μ on the electric field strength F was described by the formula μ=μ0 exp[(F/F0)1/2], where μ0 is the zero-field mobility and F0 is the characteristic field. The polymer parameters μ0 and F0 were determined by fitting the experimental data for the PMOPT-based diode to this dependence. For two PMOPT-based diode samples (with L=100 and 200 nm), the zero-field mobility was found to be μ0=2.4×10−6 cm2 V s with a characteristic field of F0=22.7 kV/cm. By comparing these results to the data for poly[2-methoxy-5-(2′-ethyl-hexyloxy))-1,4-phenylene vinylene] (PMEH-PV), it was found that the carrier mobility in PMOPT was seven times higher than that in the poly(phenylene vinylene) derivative.
Organic diodes have been also made using Schottky contacts formed on a highly doped organic semiconductor [E. J. Lous, P. W. M. Blom, L. W. Molenkamp, and D. M. de Leeuw, Schottky Contacts on a Highly Doped Organic Semiconductor, Physical Review B, 51(23), 17252–17254 (1995), incorporated herein by reference]. In these diodes, the α, α′-coupled dodecathiophene oligomers with four n-dodecyl side chains at the thiophene rings 2, 5, 8, and 11:T12d4 have been used as semiconducting materials. The T12d4 was doped with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in THF solution. The doping level was defined as the percentage of donated holes per thiophene ring. One DDQ molecule can donate two holes, so that the number of DDQ molecules is half the doping level. The diode structures were obtained from T12d4 solutions to which DDQ was added to a doping level of 5%, which corresponds to a donated hole density of about 4×1020 cm−3. The diode comprises a thin layer with low acceptor density (p−) at the metal-oligomer interface and a bulk semiconductor layer that has a higher dopant concentration (p+). The films were spin-deposited onto glass substrates preliminarily provided with four deposited Au stripe contacts for determination of the bulk conductivity σbulk. Gold forms the ohmic contact with doped T12d4. In the diode structure, Au was used as the bottom electrode, while the top Schottky contact was obtained by depositing In onto the organic film at a pressure of about 10−6 Torr. The Schottky-type diodes made of highly doped oligomers showed considerably improved current density at 1 V, JF(1 V), and had a rectification range over four orders of magnitude.
A metal-insulator-semiconductor (MIS) diode structure with poly(3-hexylthiophene) (P3HT) as the semiconductor was described by E. J. Meijer, A. V. G. Mangnus, C. M. Hart, D. M. de Leeuw, and T. M. Klapwijk [Frequency Behavior and the Mott-Schottky Analysis in Poly(3-hexylthiophene) Metal-Insulator-Semiconductor Diodes, Applied Physics Letters, 78(24), 3902–3904 (2001), hereby incoprorated by reference]. The P3HT-based MIS diodes were fabricated on glass substrates using patterned indium tin oxide (ITO) contacts as gate electrodes. A 300 nm thick insulating layer of novolak photoresist was spin-coated on top of the gate. Over the insulator, a 200 nm thick P3HT film was spun from a 1 wt. % chloroform solution. Finally, a 10 nm thick gold layer was deposited through a shadow mask to form the ohmic contact with the P3HT layer.
The Schottky diodes of another type [M. Narasimhan, M. Hagler, V. Cammarata, and M. Thakur, Junction Devices Based on Sulfonated Polyaniline, Applied Physics Letters, 72(9), 1063–1065 (1998), hereby incorporated by reference] were fabricated using aluminum/neutralized sulfonated polyaniline (SPAN) junctions. Thin films of SPAN-based materials were prepared on an indium tin oxide (ITO) coated glass substrates. Aluminum was vacuum deposited on top at a pressure of 10−6 Torr. The contact between ITO and polyaniline was ohmic, whereas the Al-polyaniline contact showed very good Schottky diode properties.
There are planar Schottky barrier diodes [M. Willander, A. Assadi, and C. Svensson, Polymer Based Devices, Their Function and Characterization, Synthetic Metals, 55–57, 4099–4104 (1993), hereby incorporated by reference] fabricated using poly(3-alkylthiophene) (P3AT) as an active semiconductor, with aluminum and gold electrodes used as the Schottky and ohmic contacts, respectively. Poly(3-alkylthiophene) used in the diodes was a p-doped semiconductor. A P3TO layer was spun onto a silicon wafer substrate and then covered by silicon oxide. Both metal contacts were formed on silicon oxide by vacuum deposition at a pressure below 10−6 Torr. Electrical characterization confirmed diode behavior with a rectification ratio greater than 104 at a sufficiently high voltage. The ideality factor was as low 1.2.
Schottky diodes have been fabricated [V. Kazukauskas, H. Tzeng, and S. A. Chen, Trap Level and Effect of Oxygen in Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] Diodes, Applied Physics Letters, 80 (11), 2017–2019 (2002), hereby incorporated by reference] using poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] as an active semiconductor, and aluminum and ITO glass electrodes as the Schottky and ohmic contacts, respectively.
In all examples of the organic diodes mentioned above, the organic layers do not possess a globally ordered crystalline structure. In addition, organic compounds used in said diodes have low values of the electric conductivity. Furthermore, the organic layers of organic diodes have been manufactured in relatively expensive production process. In particular, vacuum based deposition processes have been used to fabricate the organic components of organic diodes. These vacuum processes do not create cost advantage that would be expected from introduction of a new material in well known function. Another disadvantage of the aforementioned organic diodes is low temperature stability of the organic material itself. The desirable temperature range for diode operation in many applications is between −40 and 700° C. Present organic diodes do not satisfactorily meet these temperature requirements.